The prior art comprises different types of touch-sensitive systems that operate by transmitting light inside a solid light transmissive panel, which defines two parallel boundary surfaces connected by a peripheral edge surface. Specifically, light is injected into the panel so as to propagate by total internal reflection (TIR) between the boundary surfaces. An object that touches one of the boundary surfaces (“the touch surface”) causes a change in the propagating light that is detected by one or more light detectors. In one implementation, e.g. as disclosed in WO2008/017077, US2009/267919 and WO2010/056177, light detectors are arranged behind the panel to detect light which scatters off the touching object and escapes the panel via the boundary surface opposite to the touch surface. In another implementation, e.g. as disclosed in WO2007/003196 and U.S. Pat. No. 7,435,940, light detectors are arranged at the periphery of the panel to detect light which scatters off the touching object and is confined within the panel by total internal reflection. In yet another implementation, e.g. as disclosed in U.S. Pat. No. 7,432,893 and WO2010/064983, light detectors are arranged at the periphery of the panel to sense the attenuation of the light transmitted through the panel.
There are different approaches for injecting the light into the panel. It is known in the art to inject light along an elongate portion of the peripheral edge surface. For example, US2006/0114237, U.S. Pat. No. 6,972,753 and US2007/0075648 propose injecting directional light into the panel via the edge surface. Incoupling via the edge surface is simple but requires the edge surface to be highly planar and free of defects, at least if the light source is not attached to the edge surface. It may be undesirable to attach the light source to the edge surface, since this may impose mechanical load on soldering seams between the light source and a connecting substrate such as a PCB (Printed Circuit Board). Defect free edge surfaces may be difficult and/or costly to achieve, especially if the panel is thin and/or manufactured of a comparatively brittle material such as glass. In order to improve the strength of the panel, the edge surface may be provided with a bevel, which may further limit or obscure the incoupling of light. It may also be difficult to optically access the edge surface if the panel is attached to a mounting structure, such as a frame or bracket, and the mounting structure may cause strain in the edge surface, affecting the optical quality of the edge surface and resulting in reduced incoupling performance.
An alternative approach is to inject the light via a coupling element attached to one of the boundary surfaces of the panel to define an incoupling site.
Depending of touch-sensing technique, it may be desirable to inject light into the panel such that the light diverges in the plane of the panel as it propagates away from the incoupling site, to form a so-called “fan beam”.
Aforesaid U.S. Pat. No. 7,432,893 proposes incoupling of diverging light from a point source by means of a revolved prism which is attached to the top boundary surface of the panel. The revolved prism is designed to receive the diverging light and refract the incoming light in a direction transverse to the panel to generate reflection angles in the panel that sustain propagation by TIR, while retaining the direction of the incoming light in the plane of the panel. The revolved prism is a bulky component which may add significant weight and size to the touch system. The size of the prism also limits the number and density of the incoupling sites. To reduce weight and cost, the wedge may be made of plastic material. On the other hand, the panel is often made of glass, e.g. to attain required bulk material properties (e.g. index of refraction, transmission, homogeneity, isotropy, durability, stability, etc) and surface evenness of the top and bottom surfaces. The present Applicant has found that the difference in thermal expansion between the plastic material and the glass may cause such a prism to come loose from the panel as a result of temperature variations during operation and storage of the touch system. Even a small or local detachment of the prism may cause a significant decrease in the performance of the system.
The above discussion is equally applicable to techniques for coupling of light out of the panel. The light may be detected by light detectors directly attached to the edge surface, but this may cause the light detectors to also act as mirrors to the light in the panel at certain angles of incidence, potentially causing uncontrolled and undesirable reflections inside the panel. Alternatively, the light detectors may be directly attached to one of the boundary surfaces. In either case, this may lead to mechanical load on soldering seams between the detector and a connecting PCB. Furthermore, light detectors directly attached to the panel may be exposed to ambient light, i.e. light that originates from sources outside of the panel.
The prior art also comprises U.S. Pat. No. 7,995,039.